What are the characteristics of topography, climate and rainfall in northwest, southeast and southwest China?

Influence of Qinghai-Tibet Plateau on Climate in China;

Terrain is one of the main factors affecting climate. The Qinghai-Tibet Plateau is located in central Asia and southwest China, and is called the "roof of the world". 27 N in the south and 40 N in the north, with a longitudinal span of 13. The total area is about 2.3 million square kilometers; The average altitude is 4500 meters. Its vast territory and high terrain are incomparable to other plateaus in the world. Such a magnificent gesture not only makes it form a non-zonal plateau climate, but also plays a barrier role to the eastward advancement of the westerly airflow in the northern hemisphere and the East Asian monsoon circulation. At the same time, it also plays an important role in the formation of southwest vortex that causes heavy rain or heavy rain in eastern China.

First, in winter, the westerly belt in the northern hemisphere moves south. Due to the existence of the tall Qinghai-Tibet Plateau, the westerly airflow below 3-4 kilometers is divided into two rapids, north and south. The north branch forms southwest airflow in the northwest of the plateau, which brings certain humidity to Tianshan area in the north of the plateau and central Xinjiang. When this airflow bypasses the northern part of Xinjiang, it merges with the polar continental air mass in the south and becomes a powerful northwest airflow, which strengthens the influence of winter monsoon in China and extends far south. The south branch airflow forms a northwest airflow in the southwest of the plateau, which makes the already dry northwest of South Asia worse and drier (because of the world climate type, it belongs to the tropical desert climate). After bypassing the south side of the plateau, this airflow turns to the southwest airflow, passes through the Yunnan-Guizhou Plateau in China, and continues to move northeast until the middle and lower reaches of the Yangtze River. This warm air flow from low latitudes is often an important factor that causes the "warm winter" weather in the south of China. These two streams meet in the middle and lower reaches of the Yangtze River and flow eastward, forming the strongest westerly belt in the northern hemisphere. This westerly wind plays an important role in the weather changes in eastern China (the clouds we see on satellite images always move from west to east, and its driving force is this westerly wind). At the same time, the Sichuan Basin and Hanzhong, which are located on the east side of the Qinghai-Tibet Plateau in China, are just between the two air currents, with weak wind and stable air, becoming "dead water areas" in cloudy weather.

In summer, the westerly belt in the northern hemisphere moves northward, the airflow in the south branch of the westerly belt disappears, the summer monsoon moves northward rapidly, and cyclone activities are frequent. The monsoon region in eastern China entered the rainy season from south to north. After 10, the westerly winds gradually moved southward, the southerly westerly airflow reappeared, the summer monsoon retreated, and the winter monsoon controlled the north and south of eastern China. To sum up, without the barrier of the Qinghai-Tibet Plateau, most parts of China could be influenced by the prevailing westerlies. In that case, the climate of China would be a different scene.

Secondly, due to the obvious thermal effect produced by the Qinghai-Tibet Plateau itself, this thermal effect directly affects the monsoon circulation in East Asia. In winter, the huge high reason is high terrain, large ice and snow area, thin air, rapid radiation cooling and rapid cooling, which makes it a low temperature and high pressure center. On the one hand, this center strengthens the airflow of the south branch of the west wind on the south side of the plateau; On the other hand, this low temperature and high pressure center is superimposed on the Mongolian high pressure, which further strengthens the strength of winter winds and increases the temperature difference between the north and the south in eastern China. In summer, there is a thermal depression on the Qinghai-Tibet Plateau. This thermal depression strongly attracted the southwest warm and humid airflow from South Asia, strengthened the influence of southwest monsoon, and transported a lot of precipitation to the northern part of the Yangtze River and the Jianghuai area. Special years can also affect western Sichuan and eastern Gansu. At the same time, a warm high pressure often forms at the altitude of the plateau. When this warm and high pressure moves eastward, it will often bring dry weather to Sichuan, Shaanxi, Yunnan, Guizhou and other provinces, ending the meiyu in the middle and lower reaches of the Yangtze River and turning into a summer drought. If this warm high pressure is located to the west, it will be rainy in the middle and lower reaches of the Yangtze River, eastern Sichuan and Guizhou. However, there is less rain in western Sichuan and north China; If it is located in the north, the Yangtze River basin is less rainy and dry; To the south, the Yangtze River basin is rainy and waterlogged.

Secondly, due to the blocking effect of the Qinghai-Tibet Plateau, the exchange of cold and warm air between the north and south in the lower troposphere in western China is directly blocked. In winter, the winter wind is blocked in the northern part of the plateau, which makes the cold high pressure force in the northwest inland of China stronger and makes it difficult for cold air to move south and east. India, located in the south of the plateau, has higher temperature, lower air pressure and lower annual temperature than its eastern and western latitudes. In summer, the Qinghai-Tibet Plateau prevents the southwest monsoon from moving northward, so that a large number of warm and humid air flows from the tropical Indian Ocean can only stay in the northeast of South Asia and the southeast corner of the Qinghai-Tibet Plateau. Part of the southwest warm and humid air that swept the southeast edge of the plateau entered the southwest of China. Central China and East China have strengthened the precipitation process in these areas, while the northwest region is dry and rainless due to the barrier effect of the Qinghai-Tibet Plateau.

In addition, the southwest vortex that caused the rainstorm in eastern China is located on the Qinghai-Tibet Plateau. Due to the thermal effect of the Qinghai-Tibet Plateau, the air over the plateau can generate thermal convection, and the cloud bubbles over the plateau can gather into cloud clusters, cloud belts or cloud belts. Finally, under the influence of westerly jet in the south, they move out of the plateau in a jumping horizontal movement, causing a lot of rainfall in eastern China.